This invention relates to a process for the formulation of microencapsulated insecticides of the pyrethrin family. More particularly, a process for the microencapsulation of natural pyrethrins employing polyamide-polyurea capsule walls is disclosed which is also applicable to the microencapsulation of "synergized" natural pyrethrins. This invention is also concerned with resulting microencapsulated insecticidal materials and with processes for their use.
The pyrethrins are a class of entomologically active materials which are based upon certain chemical species which occur naturally in pyrethrin plants. Pyrethrins, whether naturally occurring or synthetic, have for some time been recognized as being insecticidal agents. A further advantage of pyrethrins as insecticides is their relatively low toxicity towards mammals and to certain other non-insect species. Thus, the pyrethrins as a class are known to be "environmentally acceptable" as insecticides.
A serious drawback to the widespread employment of pyrethrins as insecticides is the fact that they are labile toward oxidation, hydrolysis and other degradation. As a result, the entomological activity of pyrethrins decreases with time; they are, accordingly, known to be relatively non-persistent. While a lack of persistency may be beneficial in certain applications such as for use in the vicinity of domiciles, for many applications, such lack of persistency is a severe drawback. Thus, for example, the use of native pyrethrins in agriculture is generally economically unfeasible due to such lack of persistency. As will be apparent, increasing the persistency of pyrethrin insecticides is a desirable goal; progress toward this goal has been made on several fronts. Thus, certain synthetically-derived pyrethrin insecticides have been formulated which have greater resistence toward degradation and concomitantly longer persistency. Other attempts at increasing the persistency of pyrethrin pesticides have focused on partial isolation of the insecticide from the degrading effects of the environment. Thus, for example, U.S. Pat. No. 4,056,610 issued to Barber, Jr. et al discloses the formulation of microencapsulated insecticides, including pyrethrins, with polyurea microcapsules having photostabilizing ultraviolet light absorbent materials which minimize photoxidation of the encapsulated species. As will be discussed more fully hereinbelow, the traditional methods of microencapsulation such as practiced in U.S. Pat. Nos. 3,577,515 issued to Vandegaer, 3,270,100 issued to Jolkovski, 3,429,827 issued to Ruus, or 3,959,464 issued to DeSavigny, when applied to naturally-occurring pyrethrins, do not provide polyamide-polyurea microencapsulated naturally-occurring pyrethrins acceptable for use in many processes. This invention overcomes this and other shortcomings.
The process of this invention constitutes a modification of known process for the formulation of polyamide-polyurea microencapsulated materials. In this respect, reference is again made to the U.S. Vandegaer and DeSavigny patents, both assigned to the assignee of this invention, both of which patents are incorporated herein by reference. These patents provide an excellent overview of polyamide-polyurea microencapsulation techniques. The Barber patent, U.S. Pat. No. 4,056,610 which discloses an approach toward the microencapsulation of inter alia pyrethrins, is limited to polyurea-type encapsulation systems and is not directed to the polyamide-polyurea systems of the present invention. Accordingly, those skilled in the art are directed to the Vandegaer and DeSavigny patents for a fundamental understanding of interfacial polymerization techniques and methods leading to microencapsulation employing polyamide-polyurea systems.
In general, the microencapsulation technique of Vandegaer and DeSavigny is accomplished by interfacial polymerization. Purusant to this technique, a dispersed phase of a liquid containing the pyrethrin is established in a continuous phase, the dispersed phase further contains the first of at least two complimentary reactants which react together to form the capsule wall by condensation polymerization. Thereafter, the second of the complementary reactants is introduced into the continuous phase and polymerization occurs at the interface between the dispersed droplets and the continuous suspension medium. It will be understood by those skilled in the art that more than two complementary polymer-forming reactants may be incorporated in varying combinations in either of the immiscible liquid phases as long as contact between two complimentary reactants takes place only by mixing.
In general, it is desirable to form microcapsules having capsule walls which are cross-linked. As recognized by Vandegaer, such cross-linking is possible only by including one or more "polyfunctional" species in the polymer system. In this regard, polyfunctional is defined to mean "having at least three reactive functionalities per molecule." Those skilled in the art will appreciate that such reactive functionalities may be the same or different. Such persons will also appreciate that it is not necessary for all of the molecules of a constituent species to have three or more functionalities for such species to be polyfunctional. Thus a species may be said to be polyfunctional when it has in excess of two reactive functionalities on average. For example, polymethylene polyphenylisocyanate may be considered to be polyfunctionaly even though, on average, each molecule may have only about 2.6 reactive functionalities.
It is therefore, necessary for at least one of the aqueous or organic phases to comprise at least one polyfunctional species in order to secure cross-linked capsule walls. Those skilled in the art will understand that the degree of cross-linking may be controlled by control of the amount of polyfunctional species relative to the total polymerizable materials.
The first and second reactive species are chosen so as to be reactive with each other under the conditions prevailing during the reaction. As will be apparent for those skilled in the art from a review of the Vandegaer and DeSavigny patents which have been incorporated herein by reference, many choices exist for such first and second reactive species and may possible polymeric products may result from the reaction thereof. Thus, capsule walls may comprise amide (sulfonamide, etc.), urethane, urea, ester, and many other functions. For the practice of this invention, it has been found necessary to employ polyamide-polyurea systems.
It has been found that the general method of microencapsulation taught by Vandegaer and DeSavigny is relatively ineffective in providing satisfactory microencapsulated insecticidal material from naturally ocurring pyrethrins when use with polyamide-polyurea systems. Thus, the practice of the Vandegaer microencapsulation technique for polyamide-polyurea encapsulation with naturally-occurring pyrethrins yields microencapsulated products having a significant amount to "tackiness". This is manifested by a tendency of such microcapsules to "agglomerate" or to stick together in lumpy assemblages. Agglomeration of microcapsules results in an inability of such capsules to flow freely; a measure of agglomeration is the inability of substantial portions of the capsules to pass a 40 mesh screen. Thus, non-agglomerated capsules will pass a 40 mesh screen to the extent of at least 80% and more preferably 90%. Even more preferably non-agglomerated capsules will pass a 50 mesh screen to the extent of at least about 80%. Such agglomerated microcapsules are unsuitable for use as sprayable formulations especially for agricultural and structural pest control, for coating, and for other uses. While it is not entirely understood what mechanism occurs to cause the establishment of tackiness or agglomeration in such systems, it is believed that the chemical nature of naturally-occurring pyrethrins may be such that interference with the capsule wall polymerization process takes place. Alternatively, it is possible that impurities which are usually associated with naturally-occurring pyrethrins and pyrethrin preparations undertake such interference, resulting in agglomeration upon the practice of such prior microencapsulation techniques. More particularly, it is believed that naturally-occurring pyrethrins or impurities associated therewith may react with amine reactants in microencapsulation systems, thus to result in the observed agglomeration. The Vandegaer and DeSavigny patents both disclose encapsulation techniques which cause addition of amine components over a substantially instantaneous time period. It is the fast addition which is believed to facilitate agglomeration when applying those methods to the polyamide-polyurea microencapsulation of naturally-occurring pyrethrins. This invention overcomes these shortcomings and provides microencapsulated naturally-occurring pyrethrins which are at once free-flowing and non-agglomerating.